Fixed CollectionView doc reference to former OnItemAdded event callback

This commit changed the 'item:added' event to 'before:item:added' and 'after:item:added' -

https://github.com/marionettejs/backbone.marionette/commit/fade8d18d07efa5ac5d32a938f8fd857fedeb06f#src/marionette.collectionview.js

The docs described an OnItemAdded event callback: I've updated them refer to the current -  OnBeforeItemAdded and OnAfterItemAdded - event callbacks, instead.